In recent years, prefilled syringes have been frequently used, in which a liquid, such as a liquid medicine, is previously filled. Such prefilled syringes may be filled with a solvent for dissolving a medicine in a vial.
For insertion of a gasket into the prefilled syringe, a method called vacuum capping is generally used. In the method, a barrel comprising a sealed distal end opening is filled with a medicine, the gasket is disposed at an opening portion of the barrel in a reduced-pressure atmosphere (vacuum atmosphere), and then the gasket is inserted into the barrel upon increasing the reduced-pressure atmosphere to a normal pressure state. When a plunger is mounted to the gasket, it is difficult to mount the gasket to the opening portion of the barrel. Accordingly, the capping is performed on the gasket without the plunger. Therefore, in the prefilled syringe, after insertion of the gasket into the barrel, the plunger is generally mounted to the gasket. For easy mounting of the plunger to the gasket, prevention of liquid leakage during mounting the plunger, and inhibition of internal pressure rise, a mounting mechanism has been generally employed in which the plunger is provided with an external threaded portion and the gasket is provided with an internal threaded portion, and the plunger and the gasket are mounted to each other by threaded engagement.
As described above, in the prefilled syringe filled with the solvent, the plunger mounted to the gasket may be operated to be drawn for dissolving the medicine in the vial in the solvent or recovering the dissolved medicine. However, when the plunger is operated to be drawn, there is a risk that the plunger may be separated from the gasket, and the medicine cannot be dissolved or the dissolved medicine cannot be recovered.
There is known a prefilled syringe that comprises, as a mechanism for preventing such separation of the gasket, in an inner cavity portion of the gasket, a plunger retaining annular rib in the back of a helical threaded engagement portion, the helical threaded engagement portion configured for threaded engagement with a helical rib of a plunger. The helical rib of the plunger screwed in the gasket is accommodated in an accommodation portion in the back of the retaining annular rib provided in the inner cavity portion of the gasket, for prevention of separation of the gasket. Such a mechanism for preventing separation is disclosed in JP 2009-142508 A (“Patent Literature 1”) proposed by the applicant of the present application. The mechanism comprises a retaining annular rib, but a gasket is prevented from being turned when a plunger is mounted to be screwed therein, and mounting of the plunger to the gasket is facilitated.
A syringe 10 of Patent Literature 1 comprises a gasket 3, a barrel 2, and a plunger 4. The plunger 4 comprises a helical rib 44 provided on an outer surface of a head portion 42, and the gasket comprises a helical threaded engagement portion 33 threadedly engaged with the helical rib 44, a plunger-retaining annular rib 34 positioned on a distal side near the helical threaded engagement portion 33, and an accommodation portion 32 for a part of the head portion 42 of the plunger 4 at which the helical rib is formed. The retaining annular rib 34 comprises a rib absent portion 35 for guiding, to the accommodation portion, the helical rib reaching the retaining annular rib as threaded engagement is advanced between the helical rib of the plunger and the helical threaded engagement portion of the gasket. The syringe 10 is configured so that, when the plunger 4 is mounted to the gasket 3, the helical rib 44 passes the retaining annular rib 34 after passing the rib absent portion 35, and the helical rib 44 does not directly ride over the retaining annular rib 34. Therefore, strong contact between the helical rib 44 and the retaining annular rib 34 is prevented, and the gasket 3 is prevented from being turned due to the turning of the plunger 4 during mounting the plunger 4. Further, the helical rib 44 is not configured to directly ride over the retaining annular rib 34, so that mounting of the plunger 4 is also facilitated.